1. Field of the Invention
The present invention relates to a reflective encoder, which is an optical system of an optical sensor used as a part of an optical shaft angle encoder for generating an electric signal to indicate the angular position or angular change of a shaft.
2. Description of the Related Art
FIG. 11 shows a structural example of a conventional reflective encoder.
The reflective encoder is an optical encoder provided with a capsule-shaped reflective sensor 1100 that detects a modulated light beam Pa1102 that is reflected from reflective regions 1104a of a code wheel 1104. The reflective sensor 1100 includes a light emitting element 1101 that illuminates the reflective regions 1104a and non-reflective regions 1104b of the code wheel 1104; at least one light detecting element 1102 that is arranged on the same substrate as the light emitting element 1101, in order to detect the modulated light beam Pa1102 that is reflected from the code wheel 1104; a frame 1107 on which the light emitting element 1101 and the light detecting element 1102 are mounted; and an epoxy resin portion 1103 that covers the surface of both the light emitting element 1101 and the light detecting element 1102, and protects the light emitting element 1101 and the light detecting element 1102.
In order to prevent a light beam Pb1102, which is undesirably reflected at the phase boundary between the epoxy resin portion 1103 and air, from reaching directly onto the light detecting element 1102, the reflective sensor 1100 contains a lens appropriately arranged between the light emitting element 1101 and the code wheel 1104. The lens also enlarges the image towards the light detecting element 1102, and thus by using a lens it is possible to use a more compact, less expensive light detecting element 1102.
The capsule-shaped reflective sensor 1100 shown in FIG. 11 includes individual lenses, namely a light emitting lens 1105 that covers the light emitting element 1101, and a light detecting lens 1106 that covers the light detecting element 1102. The light emitting element 1101 and the light detecting element 1102 are arranged in appropriate positions such that the light beam Pa1101 from the light emitting element 1101 is enlarged by the light emitting lens 1105, and is focused and emitted in the direction of the code wheel 1104, and the modulated light beam Pa1102 reflected from the code wheel 1104 is then enlarged and focused in the direction of the light detecting element 1102. It should be noted that it is possible to use such a double lens structure, which is compact and inexpensive, provided that high accuracy is maintained.
However, as an example of such a reflective encoder, there is an optical encoder in which a light emitting device and a photodetector are enclosed within a single transparent medium (see for example, JP H6-221874A (1994)).
The reflective encoder shown in FIG. 11 provides a number of advantages over reflective encoders that have been used up to now, namely being relatively inexpensive, and relatively compact, however there are problems, such as are indicated below, because the light emitting element and the light detecting element are both provided within the same transparent medium.
That is to say, the light beam Pb1101 from the light emitting element 1101 is internally reflected by the epoxy resin portion 1103 that protects the light emitting element 1101, and is irradiated as the light beam Pb1102 onto the adjacent light detecting element 1102. Thus, an undesired signal is generated in the light detecting element 1102.
FIG. 12 is a graph showing an example of an output waveform when an undesired signal is generated in the light detecting element, and FIG. 13 is a graph showing an example of an output waveform when an undesired signal is not generated in the light detecting element. In FIG. 12 and FIG. 13, the vertical axis indicates voltage, and the horizontal axis indicates time.
As illustrated, the output signal waveform when an undesired signal is generated in the light detecting element is shifted upward by a noise component N, when compared to the output signal waveform when the undesired signal is not generated in the light detecting element.
In reflective encoders, the degree of accuracy of signal detecting has a great influence on the performance of the reflective encoder. Thus, precision loss due to internal reflection is a significant problem. Also, in order to overcome such internal reflection, it is necessary to use a relatively high current for emitting light, leading to an increase in power use. Furthermore, from the result of experiments, it has been found that the undesired signal that directly enters the light detecting element from the light emitting element induces a noise component shift in the output signal waveform that is about ⅙ the amplitude of the output signal waveform. Therefore it is necessary to remove the noise component.
As above, the problem of the effect due to internal reflection is greater with reflective encoders than with reflective photo interrupters. As shown in FIG. 13, in order to improve performance, it is important that there is substantially no noise component in the output signal waveform.
On the other hand, there is also the problem that if the distance between the lens of the light emitting element and the lens of the light detecting element is large, then the amount of light emitted by the light emitting element that reaches the light detecting element is reduced.